A four-year XMM-Newton/Chandra monitoring campaign of the Galactic Centre: analysing the X-ray transients

Abstract

We report on the results of a four-year long X-ray monitoring campaign of the central 1.2 square degrees of our Galaxy, performed with Chandra and XMM-Newton between 2005 and 2008. Our study focuses on the properties of transient X-ray sources that reach 2-10 keV luminosities of >1E34 erg/s for an assumed distance of 8 kpc. There are 17 known X-ray transients within the field of view of our campaign, eight of which were detected in outburst during our observations: the transient neutron star low-mass X-ray binaries GRS 1741-2853, AX J1745.6-2901, SAX J1747.0-2853, KS 1741-293 and GRO J1744-28, and the unclassified X-ray transients XMM J174457-2850.3, CXOGC J174535.5-290124 and CXOGC J174541.0-290014. We present their X-ray spectra and flux evolution during our campaign, and discuss these results in light of their historic activity. Our main results include the detection of two thermonuclear X-ray bursts from SAX J1747.0-2853 that were separated by an unusually short time interval of 3.8 min. We detected a thermonuclear X-ray burst and a ~1600-s X-ray eclipse from AX J1745.6-2901. Both XMM J174457-2850.3 and GRO J1744-28 displayed weak X-ray activity above their quiescent levels that is indicative of low-level accretion. In addition to the eight known X-ray transients, we discovered a previously unknown X-ray source that we designate XMMU J174654.1-291542. Based on its X-ray properties and the possible association with an infrared source, we tentatively classify this object as a cataclysmic variable. No new transients were found during our campaign, reinforcing the conclusion of previous authors that most X-ray transients recurring on a time scale of less than a decade have now been identified near the Galactic centre.

25 pages, 13 figures, 4 tables, A&A in press, corrected an error in one source name (XMMU J174654.1-291542)

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• The peculiar Galactic center neutron star X-ray binary XMM J174457-2850.3
• Numerical simulations of axisymmetric hydrodynamical Bondi-Hoyle accretion onto a compact object
• Very-high energy observations of the Galactic center region by VERITAS in 2010-2012
• New insights on the puzzling LMXB 1RXS J180408.9-342058: the intermediate state, the clocked type-I X-ray bursts and much more
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• XMM-Newton observations of the Galactic Centre Region - II: The soft thermal emission
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• Broadband X-ray properties of black holes GRS 1758-258 and 1E 1740.7-2942: AstroSat and NuSTAR results
• The quiescent counterpart of the peculiar X-ray burster SAX J2224.9+5421
• The advection-dominated accretion flow for the anti-correlation between the X-ray photon index and the X-ray luminosity in neutron star low-mass X-ray binaries
• Monitoring the X-ray Variability of Bright X-ray Sources in M33
• Unveiling the soft X-ray source population towards the inner Galactic disk with XMM-Newton
• The very faint X-ray transient Swift J174610-290018 at the Galactic center
• A peculiar thermonuclear X-ray burst from the transiently accreting neutron star SAX J1810.8-2609
• Recurrent Instability in LMXB Accretion Disks: How Strange is GRS 1915+105?
• High Energy Emission from the Galactic Center
• Is the Peculiar Galactic Center Transient Swift J174610.4-290018 A Nova Outburst?
• NuSTAR and AstroSat observations of thermonuclear X-ray bursts with short-recurrence times in 4U 1636$-$536